A controlled double blind study of immunotherapy of cat induced asthma in comparison with placebos will be carried out. Responses will be evaluated by clinical symptoms and airway responses to aerosol of cat pelt extract, methacholine and exposure to live cats. Immunological responses will be assessed by measurement of basophil histamine release, IgG antibodies, IgE antibodies to cat pelt extract and by assay for circulating immune complexes and for pemphigus antibodies. Allergens of cat pelt extracts will be identified and purified.